The present invention relates generally to the operation of alternating current motors and, more particularly, to a protection system for alternating current propulsion systems for battery powered vehicles.
The Government of the United States of America has rights in this invention pursuant to Contract Number DE-AC07-90ID13019 awarded by the U.S. Department of Energy.
Battery powered vehicles have generally utilized direct current (DC) electric motors for propulsion. Recently, the use of alternating current (AC) motors for such applications has become possible, taking advantage of the better power-to-weight ratio of AC motors along with improved efficiency and reduced maintenance requirements. Some of these vehicles have begun to rival or exceed the performance of equivalent internal combustion powered vehicles. However, the higher power density of the AC motor propelled vehicles leads to higher power dissipation and rapid temperature rises during transient operation at peak power. It is therefore desirable to provide an effective protection system for components in the propulsion system of such vehicles.
One example where protection is needed is in the use of a high speed AC motor incorporated in a transaxle assembly for propelling a vehicle. The motor is normally sprayed with transmission fluid for cooling and can generate about one hundred horsepower from a 130 pound motor for intermittent acceleration or hill climbing. Such a motor would normally be called upon to produce about 15 horsepower to maintain 60 miles per hour on level terrain. If the motor is operated at peak power for an extended period, it may overheat and fail. Further, if motor speed drops significantly under such loading, the cooling fluid may not be adequately sprayed over the motor resulting in a higher rate of heating. The slower speed of the motor may further result in higher currents being drawn and risking damage to the electronic components supplying power to the motor.